First of all, lets clear one thing up about leggings. Leggings are OPAQUE. If they are see though, then they are footless stockings, which can't be worn as pants, and are just hideous. Leggings should also hit at the ankle. Outside of being skin tight (yay!) they should be like normal trousers.
Can Leggings be worn as trousers? I think so. So long as the fabric is thick enough and you're wearing them with a cool top, it looks good.
Jeggings (denim print leggings) are meant to be worn just as you wear your jeans. I like the idea of this, you get the look of a skinny jean and the comfort of a legging.
Some jeggings I don't really like because they look cheap or don't look enough like jeans. The above pair by Juicy Couture are really cute, but I like the jeggings with the fake zippers/buttons better. Such as these:
I'm really loving the detail that being put on leggings now. I feel like some of the cooler looking leggings can be the focus of my outfit. Which makes getting dressed a lot easier.
I love the detail on these, and they would look awesome with a black or white top,
These have to be some of the best ones I've seen. I would feel like I'm wearing water! I think they would look cool with a longer blazer (with shoulder pads, natch) and sky high heels,
Now of course, accessories and the other parts of the outfit should be kept to a minimum. Too much of a good thing is just to much, and there is a fine line between tacky and trendy.